1. Field of the Invention
The present invention relates to a motor control device configured to output a control signal for controlling a motor, while suppressing natural vibrations of a target object constituted of the motor and a driven member to be driven by the motor.
2. Description of the Related Art
Conventionally, a motor control device provided with a bandstop filter configured such that a center frequency corresponding to a natural frequency of a target object is set in order to suppress natural vibrations of the target object constituted of a motor and a driven member to be driven by the motor in a machine tool, an industrial machine or a robot using the motor has been proposed.
Use of a bandstop filter in a motor control device makes it possible to remove a frequency component corresponding to a natural frequency of a target object from a control signal (e.g. a torque command) for controlling a motor. This makes it possible to control the motor, while suppressing natural vibrations of the target object.
When the spring constant of a ball screw is changed in accordance with the relative position between a motor and a driven member in driving the driven member by the motor via the ball screw along a linear moving axis of a machine tool, the natural frequency of the target object is changed in accordance with the position of the driven member being moved.
When the natural frequency of the target object is changed as described above, it may be impossible to suppress natural vibrations of the target object, because the frequency corresponding to the natural frequency of the target object is out of the bandwidth of the bandstop filter.
For instance, as described in Japanese Patent No. 4,238,273, a motor control device provided with a variable bandstop filter whose center frequency is variable in order to suppress natural vibrations of a target object even when the natural frequency of the target object is changed in accordance with the position of a driven member being moved has been proposed (see e.g. Patent Literature 1).
A conventional motor control device provided with a variable bandstop filter as described above is configured to extract a magnitude (power spectrum) of a frequency component included in a control signal at a predetermined frequency step size, and to detect a frequency corresponding to the natural frequency of a target object from the extracted magnitude of the frequency component.
The conventional motor control device is configured such that when the center frequency of the variable bandstop filter is changed by a change value equal to an integral multiple of 1 or more of the frequency step size resulting from a change in the natural frequency of the target object, the center frequency of the variable bandstop filter is made to coincide with the frequency corresponding to the natural frequency after the change by adding or subtracting a change value to or from the center frequency of the variable bandstop filter all at once.
Adding or subtracting a change value in accordance with a change in the natural frequency of a target object as described above makes it possible to appropriately change the center frequency of the variable bandstop filter in accordance with the position of the driven member being moved.
The predetermined frequency step size for use in extracting a frequency component included in a control signal in the conventional motor control device is set to a relatively large value (e.g. 10 Hz) when speedy detection of natural vibrations is prioritized to accurate detection of natural vibrations, and is set to a relatively small value (e.g. 1 Hz) when accurate detection of natural vibrations is prioritized to speedy detection of natural vibrations in a condition of a same frequency component extraction range and a same frequency component extraction period (a sampling period when a sampling value is subjected to discrete Fourier transform).
A smallest change amount of the natural frequency detectable by a motor control device increases, as the frequency step size increases. Therefore, the change amount per unit time between a control signal output from the variable bandstop filter before the center frequency of the variable bandstop filter is changed, and a control signal output from the variable bandstop filter after the center frequency of the variable bandstop filter is changed increases, as the frequency step size increases.
Therefore, when the center frequency of the variable bandstop filter is changed in accordance with the position of the driven member being moved, a change between a control signal output from the variable bandstop filter before the center frequency of the variable bandstop filter is changed, and a control signal output from the variable bandstop filter after the center frequency of the variable bandstop filter is changed may cause fluctuations in a control signal output from the variable bandstop filter.
The fluctuations in a control signal output from the variable bandstop filter increase, as the change amount per unit time between a control signal output from the variable bandstop filter before the center frequency of the variable bandstop filter is changed, and a control signal output from the variable bandstop filter after the center frequency of the variable bandstop filter is changed increases.
A mechanical shock increases, as the fluctuations in a control signal output from the variable bandstop filter increase, which may adversely affect the processing precision in performing processing by a machine tool.
In view of the above, there is a demand for a configuration of reducing fluctuations in a control signal output from a variable bandstop filter, which may occur when the center frequency of the variable bandstop filter is changed, when speedy detection of natural vibrations is prioritized to accurate detection of natural vibrations, specifically, when it is necessary to set a predetermined frequency step size for use in extracting a frequency component included in a control signal to be a relatively large value.